Machines for treatments such as the ones described above are used for operating blood treatment processes in patients suffering from partial or total kidney failure. Blood treatment apparatus typically comprise an extracorporeal circuit which is provided with at least one blood treatment unit, as well as at least one access branch destined to connect a blood removal zone with a first chamber of the treatment unit; the extracorporeal circuit also comprises a second branch i.e. a blood return branch which develops downstream of the treatment unit from the first chamber, towards a blood return zone to the patient.
There is usually a peristaltic pump located at the access branch, which pump is destined to act on the access branch in order progressively to move the blood flow towards the treatment unit.
Independently of the type of treatment to be performed on the patient, a precise knowledge of the quantity of blood taken from the patient is of maximum importance, being also the quantity treated by the machine the extracorporeal blood circuit is connected to.
It is important to note that the blood flow which can be obtained through the use of peristaltic pumps depends on various factors, mainly:                the geometry and material of the particular tract of blood line to which the peristaltic pump is associated;        the geometry of the pump rotor, as well as the angular velocity of the peristaltic pump;        the geometry of the tract of tubing upstream of the pump, and the access organ used for removing blood from the patient;        the pressure existing, in particular, in the tracts of tubing upstream and downstream of the peristaltic pump;        the physical-chemical characteristics of the blood.        
In a first prior-art solution used in the past, the flow produced by the peristaltic pump was considered proportional, through a special conversion factor, to the instant angular velocity of the pump itself.
In other words, in order to obtain a theoretical flow value through the pump segment, the angular velocity of the pump was multiplied by a constant calibration factor. The theoretical flow value obtained could be viewed on a display on the machine, or not as the case required.
However, considering the numerous factors briefly mentioned above that influence the entity of the flow really provided by the peristaltic pump, it is easy to see how a flow calculation using a simple proportional factor for the angular velocity is affected by errors which cannot be viewed as irrelevant. With this aim, i.e. to realise an apparatus for blood treatment in which a flow value through the pump tract of the extracorporeal circuit as close as possible to reality were known, U.S. Pat. No. 5,733,257 describes a calibration process for a peristaltic pump destined to be used with an apparatus internally provided with at least a flow meter. The process involves introducing a fluid into the pump tubing segment and activating the peristaltic pump at a constant revolution count. Once regularly revolving at the determined velocity, the pressure upstream of the pump tubing segment is read, as is the fluid flow effectively crossing the pump tract, using the flow meter internal of the machine, and in this way a pair of calibration readings is obtained (effective flow, arterial pressure) according to the pre-selected angular velocity of the pump.
The above-described process is repeated, varying the arterial pressure upstream of the pump, so as to obtain various arterial pressure-effective flow pair readings for a same angular velocity. At this point a calibration curve is calculated, thanks to which a pressure-effective flow relation is created for the selected angular velocity. By repeating the above-described calibration criteria over a range of angular velocities, a calibration curve set is created. When the machine is started up, the calibration curves are used to calculate the effective flow of the peristaltic pump on the basis of the known and measured values of the pump angular velocity and the pressure level in the tube tract upstream of the pump.
A further process for determining and controlling the blood flow is described in patent WO03/055542. This process involves determining a function of calibration F in the following variables:                v1 in relation to the angular velocity of the pump (ω),        v2 in relation to the arterial pressure (Part) in the portion of the access branch upstream of the peristaltic pump        v3 in relation to an effective blood flow (Qactual) crossing the access branch        v4 in relation to the time that has passed since start of treatment        v5, in relation to the geometrical characteristics of an access organ which is operatively couplable to the extracorporeal circuit;        v6, in relation to the length of the tract of tube of the access branch upstream of the peristaltic pump.        
The process periodically calculates the effective flow Qactual thanks to this function and, as it knows the variables from v1 to v6, the process if necessary modifies, at time intervals, the angular velocity of the peristaltic pump if the difference (Qactual−Qset) between effective flow and desired flow is out of a prefixed interval.